Process for the electrolytic production of cromium of a high degree of purity



1966 E. A. BRANDES ETAL 3,259,560

PROCESS FOR THE ELECTROLYTIC PRODUCTION OF CHROMIUM OF A HIGH DEGREE OF PURITY Filed July 20, 1962 COPPER TUBE 200 AMP 3 PH CATHODE RECTIFIER HM,

PVC STOPPING PRECIOUS METAL ANODE COATED WITYH Pbo LAGGING HIGH DENSITY POLYETHYLENE ELECTROLYTE SATURATED WITH PbCr0 TEMP. 97C

CURRENT DENSITY 460 AM P/FT Inventors 62.2 144 3AM y v Attorney United States Patent Office 3,259,560 Patented July 5, 1966 3,259,560 PROCESS FOR THE ELECTROLYTIC PRODUCTION OF CHROMIUM OF A I-HGH DEGREE OF PURITY Eric A. Brandes, Chalfont St. Peter, and James A. Whittaker, Slough, England, assignors to National Research Development Corporation, London, England, a corporation of Great Britain Filed July 20, 1962, Ser. No. 211,246 Claims priority, application Great Britain, July 26, 1961, 27,06 1/ 61 2 Claims. (Cl. 204-105) This invention relates to a process for the electrolytic deposition of chromium of a high degree of purity.

It has heretofore been proposed in the electrolytic deposition or electro-winning of chromium to use fluoride baths containing 250 g./litre CrO and 5 g./litre HF and lead alloy anodes, the most favoured lead alloys being those with a tin constituent, for example Fab/7%v Sn. The chromium obtained using such anodes has been contaminated in some degree by tin and could not be used in circumstances requiring chromium of a high degree of purity. Pure lead is not satisfactory in that it is attacked by the electrolyte, particularly when this operates at high temperature.

To obtain chromium free from nitrogen and oxygen it is necessary to use the electrolyte at high temperature, preferably as nearly as practicable to the boiling point of the electrolyte, to reduce the solubility of these gases in aqueous solution. The fluoride bath has the advantage that its efiiciency increases with increase in temperature Other baths used for the production of electrodeposited chromium, notably the sulphate bath containing 250 g./litre CrO and a CrO E) S0 1 prises producing same electrolytically in a fluoride bath using an anode of a metal or alloy that is not attacked :by the electrolyte, and adding lead to the bath and/or coating the anode with lead oxide (1 b0,).

Another object of the present invent-ion is a process of winning chromium of a high degree of purity which comprises producing same electrolytically in a fluoride bath using an anode of a metal or alloy that is not attacked by the bath, adding a lead salt to the bath and/0r coating the anode 'with lead oxide.

The metal of the anode may be platinum or palladium or an alloy containing substantial quantities of platinum or palladium.

Where lead is added to the bath, the lead is preferably in the form of a salt such as lead chromate (PbCrO Where the noble metal anode is coated with lead oxide, such coating may be eflected by anodic treatment in a saturated lead fluoride (PbF solution.

The fluoride bath is preferably in a vessel or container of high density polythene or P'DFE (poly-tetrafluoroethylene).

In the single figure of the accompanying drawing illustrating one form of apparatus suitable for use in carrying out the process of the present invention and as shown by the legends thereon, the fluoride bath electrolyte is contained in a lagged high density polyethylene tank. A

precious metal anode coated with lead oxide is partially immersed in the electrolyte bath and a copper tube cath ode is disposed interiorly of the anode and spaced therefrom. Appropriate wiring leads from the upper ends of the anodes through a rectifier to a source of alternating current. At its lower end and intermediate its length, the cathode is capped with and encased in polyvinyl chloride 7 stopping material.

The following two tables are exemplary of a series of runs in the electrolytic production of chromium from a solution of chromic anhydride (CrO in hydrofluoric acid (HF).

TABLE 1 [Solution composition 250 g./1. C103; 5 g./l. HF.]

Cathode Duration Bath Cathode Run No. 58 litres of solution in a 60 litre polythene tank Current of Run in Temp, Current Density, Hours C. Efficiency, ampsJlt. percent;

1 New solution uncoated palladium anode. No 460 24 90 30 lead additions to bath.

TABLE 2 [Solution composition 250 g./l. CrOa; 5 g.ll. HR]

Cathode Duration Bath Cathode Run No. 9 litres of the solution in a 10 litre PTFE beaker Current of Run in Temp, Current Denslty, Hours Efiiciency, amps/ft. Percent 1 New solution uncoated palladium anode. No 460 6 29 lead additions to bath. 2 do 520 6 23 3 do 410 6 15 It will-be apparent from Tables 1 and 2 that when a new solution is used with no load either on the anode or in the bath, the initial results are quite good but there is a rapid falling off in efiiciency in subsequent runs.

The following three tables are exemplary of a series of runs made with apparatus according to the present invention as exemplified in the accompanying drawing, and utilizing the method according to the present invention, in the electrolytic production of chromium of a high degree of purity from a solution of chromic anhydride (CrO in hydrofluoric acid (HF). As regards Tables 3 and 4, the high density polythene tank of the apparatus shown in the drawing is replaced by a PTFE.

normal for the analar grade of chromic acid, as it has been found that the nitrogen content of the electrolyte, particularly if present as a nitrate or nitrite is reflected in the nitrogen content of the chromium deposited on the cathode, and in the continuing efficiency of the process after a number of runs. The nitrogen content in the deposited chromium was found to be about 10 p.p.m. with chromic acid of 12 p.p.m. nitrogen content.

The pure electrodeposited chromium deposited in the manner described may be used as a decorative or protective coating or after removal from the cathode, may be used to produce chromium based alloys for use at high temperatures or stainless steel or other ferrous alloys beaker. having extremely low carbon, oxygen and nitrogen con- TABLE 3 [Solution composition 250 g./l. CrOa', 5 g./l. HR]

Cathode Duration Bath Cathode Run No. 9 litres of the solution in a 10 litre PIFE beaker Current of Run in Temp, Current Density, Hours C. Efficiency, amps/ft. Percent 1 Palladium anode. Bath saturated with lead 435 4.25 95 31 chromate (PbCrO 2 do 435 6 95 32 435 5. 5 95 27 460 6 95 22 TABLE 4 [Solution composition 250 g./l. CIOa; 5 g./l. IIF.]

Cathode Duration; Bath Cathode Run No. 9 litres of the solution in a 10litre PTFE beaker Current of Run in Temp, Current Density, Hours C. Ethciency, amps/it. Percent 1 Palladium anode. Bath saturated with 460 6 95 26 PbCrO; and anode coated with P130; by anodic treatment in saturated lead fluoride (PbFz) solution TABLE 5 Solution composition 250 g./l. CrOa; 5 g./1. HR]

Cathode Duration Bath Cathode Run No. litres of solution in a high-density poly- Current of Run in Temp., Current thene tank Density Hours C. Efficiency,

amps/ft. percent 1 Palladium anode. Bath saturated with 460 24 95 34 PhCrO; and PbOz formed on the anode by gnodic treatment in saturated PbFz solu- Spectrographic examination of the chromium yield in all cases failed to show the presence of any lead in the chromium.

It will be manifest from a comparison of Tables 3 to 5 that best results can be expected when lead is present a nitrogen content of 12 p.p.m. instead of the 40 p.p.m.

tents. Examples of such alloys are alloys with iron containing up to 40% chromium.

We claim:

1. In an electrolytic process for the production of high purity chromium from a fluoride bath of dissolved chromium low in nitrogen the improvements comprising:

(a) carrying out the electrolysis at a temperature of at least C.,

(b) carrying out the electrolysis with an anode of material selected from the group consisting of platinum, platinum alloys, palladium and palladium alloys,

(0) carrying out the electrolysis with lead chromate present in the bath, and

(d) maintaining a coating of lead dioxide on said anode material.

5 6 2. In an electrolytic process, according to claim 1, the 2,766,202 10/1956 Estes 204242 further improvements comprising: 2,787,588 4/1957 Stareck et a1. 204 51 '(a) carrying out the electrolysis at a cathode current 2,969,314 1/1961 Coxe 204-242 :ggsity of from 435 to 460 amperes per square foot 5 FOREIGN PATENTS (b) carrying out the electrolysis in a bath having a 585,846 11/1959 Canadaratio of volume in litres of said bath to the area of OTHER REFERENCES cathode m square Inches of'at least Morisset, Paul e: al.: Chromium Plating, Roher Draper Ltd., page 207, 1954. References Cited by the Examiner 11) UNITED STATES PATENTS JOHN H. MACK, Primary Examiner.

1,544,451 6/1925 H mbue h 204-51 JOHN R. SPECK, MURRAY TILLMAN, Examiners. 2,307,551 1/1943 Tnska 204-51 G. KAPLAN, Assistant Examiner.

2,704,273 3/1955 YO-shida 20451 15 

1. IN AN ELECTROLYTIC PROCESS FOR THE PRODUCTION OF HIGH PURITY CHROMIUM FROM A FLUORIDE BATH OF DISSOLVED CHROMIUM LOW IN NITROGEN THE IMPROVEMENTS COMPRISING: (A) CARRYING OUT THE ELECTROLYSIS AT A TEMPERATURE OF AT LEAST 85*C., (B) CARRYING OUT THE ELECTROLYSIS WITH AN ANODE OF MATERIAL SELECTED FROM THE GROUP CONSISTING OF PLATINUM, PLATINUM ALLOYS, PALLADIUM AND APLLADIUM ALLOYS, (E) CARRYING OUT THE ELECTROLYSIS WITH LEAD CHROMATE PRESENT IN THE BATH, AND (D) MAINTAINING A COATING OF LEAD DIOXIDE ON SAID ANODE MATERIAL. 